The present invention is related to method for operating an electric car drive controller which vector controls an induction motor.
A known arrangement for vector control of an induction motor is disclosed in the patent journal Japanese Patent Publication 62-12754. According to the method disclosed in the patent, when an induction motor is operated at a constant output power in a speed range above the base speed, increase of the induction motor voltage while the motor is operated at a speed above the base speed is controlled by using a function generator, which weakens a magnetic flux command generating means in inverse proportion to the rotating speed.
The Japanese Patent Application 62-233002 discloses another method, which calculates an optimum motor magnetic flux from the required target torque and required number of revolutions of an induction motor to minimize the overall motor loss. In order to minimize power consumption, it supplies a primary current that allows the motor loss to be the lowest at any time in the course of and after adjustment of the motor magnetic flux and torque current.
In the prior art such as disclosed in the aforesaid patent journal Japanese Patent Publication 62-12754, the magnetic flux command is intensified regardless of the strength of the output torque when the rotating speed is below the base speed. Accordingly, excessive magnetic fluxes are always generated in the induction motor even in case of small output torque, thus possibly causing a decrease in efficiency.
In addition, because of output power limitation of the electric power converter, it is necessary to employ a so called weak magnetic field control in the high speed range by further lowering the output characteristic below the constant output power, so that the induction motor voltage should not increase in the high speed operation. For this reason, a control method for weakening the magnetic flux command in correspondence to the rotating speed is employed using a function, but no consideration is given to increasing the efficiency.
Because the method requires that a voltage in excess of the capacity of the electric power converter be applied to the motor in high speed operation, it is understood that insufficient consideration is given to size reduction of the electric power converter which drives the induction motor.
On the other hand, charge state of the power source is not considered in the prior art disclosed in the patent journal Japanese Patent Application 62-233002. Because a battery, for example, is used as the power source for an electric vehicle, the residual battery charge varies drastically during running. When charge state of the power source is high, it is possible to apply a voltage to the induction motor which is higher than the design voltage (within the limit of the capacity of the electric power converter), to reduce copper loss of the motor and loss in the electric power converter, and to improve the overall efficiency of the controller. No consideration is given to this issue in the prior art.